Pope Francis inaugurates his papacy at a Mass in Rome, calling on global leaders and all the people of the world to defend the poor and the environement.

World leaders and hundreds of thousands of people celebrated the pope's inaugural mass at St Peter's Square in Vatican City.

Up to 200,000 people attended the Mass in St Peter's Square, which began with a tour of the famous Vatican plaza by Jorge Mario Bergoglio, 76, after his election last week.

Six sovereigns, U.S. Vice President Joe Biden and other leaders as well as heads of many other faiths were present among the 130 delegations on the steps of the famous basilica.

Malta was represented by President George Abela, Prime Minister Joseph Muscat and Opposition leader Lawrence Gonzi.

The new Pope's homily focused on protection - of the environment, children, the elderly and those in need, who he said were "often the last we think of".

Pope Francis' homily at the Mass began by focusing on Joseph and his role as protector - of Mary, Jesus and the Church.

Francis, 76, expanded the image, referring to Francis of Assisi and saying that the role of protector was not just a Christian one.

He said: "It means protecting all creation, the beauty of the created world... It means respecting each of God's creatures and respecting the environment in which we live.

"It means protecting people, showing loving concern for each and every person, especially children, the elderly, those in need, who are often the last we think about."

Francis called on "all those who have positions of responsibility in economic, political and social life" to be protectors of creation.

"To protect creation, to protect every man and every woman, to look upon them with tenderness and love, is to open up a horizon of hope," Francis said.

Without care for the environment and fellow humans, "the way is opened to destruction and hearts are hardened", he said.

"Let us not allow omens of destruction and death to accompany the advance of this world."

Francis said the pope himself must be inspired by the lowly - "the poorest, the weakest, the least important, those who Matthew lists in the final judgment on love: the hungry, the thirsty, the stranger, the naked, the sick, those in prison".

Despite criticism at home for allegedly failing to speak out against the excesses of Argentina's military rule, the former archbishop of Buenos Aires has already won hearts in Rome with a disarmingly informal style.

Francis had begun the day by touring St Peter's Square in an open-topped Popemobile.

At one point he stepped down from the vehicle and approached the barriers to bless a disabled man.

Francis spent 20 minutes touring the square, waving to the pilgrims who flew flags and shouted: "Long live the Pope!"

Bergoglio was a surprise choice at a conclave of cardinals to replace 85-year-old Benedict XVI, who last month brought a sudden end to a papacy that had often been overshadowed by scandal, saying he was too old to carry on.

The jovial Francis has said he chose his papal name in honour of the medieval Italian saint St Francis of Assisi and has called for a "poor Church for the poor," warning the world's cardinals against pursuing worldly glories.

Bergoglio, the son of an Italian emigrant railway worker from a working-class quarter of Buenos Aires, has been effusive in a way that is unusual in the Vatican, kissing pilgrims and doing impromptu walkabouts.

The Vatican has said security guards will have to adapt to the new style.

The arrivals have already presented Francis with a first diplomatic problem in the form of a request from compatriot President Cristina Kirchner of Argentina to mediate in a dispute with Britain over the Falkland Islands.

The Chinese government did not send any representatives after Taiwanese President Ma Ying-jeou said he was attending.

Zimbabwean President Robert Mugabe also flew in, sidestepping an EU travel ban over human rights abuses in his country that does not apply to the Vatican.

Latin America was heavily represented at the mass by the first non-European pope in nearly 1,300 years, with the presidents of Argentina, Brazil, Chile, Costa Rica, Mexico and Paraguay all expected to attend.

The Vatican is well secured for the event, with 3,000 officers deployed including snipers on the rooftops and bomb disposal experts.

The day's events began with a tour of St Peter's Square by the new pope, after which Francis prayed at the tomb of St Peter, who is considered the first pontiff in Catholic tradition.

Church leaders have urged Francis to move quickly to reform the intrigue-filled Roman Curia, the central administration of the Roman Catholic Church, and his appointments in the coming weeks will be closely watched.

Francis has indicated that he will press for a friendlier faith that is closer to ordinary people and for social justice, although the moderate conservative is unlikely to change major tenets of Catholic doctrine.